1. Field of the Invention
This invention relates to a switching power supply and more particularly to a switching power supply of a self-excited ringing choke converter (hereinafter referred to as RCC) type.
2. Description of the Related Art
Generally, electronic equipment such as electronic computers and communication devices require stable DC voltages. In order to supply stable DC voltages for such electronic equipment using the commercially available power source, switching power supplies of RCC type which has a relatively simple construction and shows high degrees of efficiency are widely used.
FIG. 7 is the circuit diagram of such a conventional switching power supply of RCC type. In FIG. 7, a switching power supply 1 is provided with an input circuit 2, a main operating circuit 3, a voltage detecting circuit 4, a voltage output terminal OUT, and a ground terminal GND. The input circuit 2 comprises a diode bridge circuit DB for rectification, and a fuse F and a filter circuit LF both of which are provided between an AC power supply and the input terminal of the diode bridge circuit DB.
Furthermore, the main operating circuit 3 comprises a capacitor C1 for smoothing provided between the output terminals a and b of the diode bridge DB in the input circuit 2; a transformer T containing a primary winding N1, a secondary winding N2 having the opposite polarity to that of the primary winding N1, and a feedback winding having the same polarity as the primary winding N1; a FET Q1 as a switching element connected in series to one end of the primary winding N1 of the transformer T; a resistor R1 for starting connected between the other end of the primary winding N1 and the gate as a control terminal of the FET Q1, a resistor R8 connected between the gate and source of the FET Q1, a diode D1 for rectification connected in series to one end of the secondary winding N2 of the transformer T, and a capacitor C4 for smoothing connected between the other end of the secondary winding N2 and the voltage output terminal OUT.
Furthermore, the voltage detecting circuit 4 is provided on the output side of the main operating circuit 3 and contains a resistor R5, a light-emitting diode PD on the emission side of a photo coupler PC, a shunt regulator Sr, and resistors R6 and R7. The resistor R5, the light-emitting diode PD, and the anode and cathode of the shunt regulator are connected in series and are provided so as to be parallel to the capacitor C4 of the main operating circuit 3. Furthermore, the resistors R6 and R7 are connected in series and are also provided to be parallel to the capacitor C4. The connection point of the resistors R6 and R7 is connected to the control terminal of the shunt regulator Sr.
Furthermore, a control circuit 5 comprises a resistor R9 and a capacitor C3 connected between one end of the feedback winding NB and the gate of the FET Q1, a transistor Q2 connected between the gate and source of the FET Q1, a resistor R2 connected between one end of the feedback winding NB and the base of the transistor Q2, a resistor R3 and a capacitor C2 connected in parallel between the base and emitter of the transistor Q2, and a resistor R4, a diode D2, and a phototransistor PT on the light-receiving side of a photo coupler PC connected in series between one end of the feedback winding NB and the base of the transistor Q2.
Next, the operation of the switching power supply 1 thus constructed is described. First of all, at start, a voltage is applied to the gate of the FET Q1 through the resistor R1 and the FET Q1 is turned on. When the FET Q1 is turned on, the voltage of the power supply is applied to the primary winding N of the transformer T, a voltage in the same direction as the voltage generated in the primary winding N1 is generated in the feedback winding NB, and then the FET Q1 is rapidly turned on because of positive feedback. At this time, excitation energy is stored in the primary winding N1.
When the capacitor C2 is charged through the resistor R2 and the potential of the base of the transistor Q2 reaches the threshold, the transistor Q2 is turned on and the FET Q1 is tuned off. Because of this, the excitation energy stored in the primary winding N1 of the transformer T, while the FET Q1 is turned on, is discharged as electric energy through the secondary winding N2, and the electric energy is rectified by the diode D1, smoothed by the capacitor C4, and supplied to a load through the voltage output terminal OUT.
Furthermore, when the excitation energy stored in the primary winding N1 of the transformer T is discharged through the secondary winding N2, a flyback voltage VNB is generated in the feedback winding NB. The change of this flyback voltage VNB is described with reference to FIG. 8. In FIG. 8, at the time t11, the FET Q1 is turned off and the flyback voltage VNB is kept at a nearly constant value Vb, that is, it goes into a so-called off-state period. Then, at the time t12, the voltage of the diode D1 becomes zero and the flyback voltage starts to oscillate, and when the flyback voltage VNB rises and the gate voltage reaches the threshold Vth at t13, the FET Q1 is turned on. Moreover, part of the flyback voltage VNB shown by the chain line shows the case where the flyback voltage VNB is assumed to continue to oscillate. In this way, when the FET Q1 is turned off, a voltage is applied to the primary winding N1 again and the excitation energy is stored.
In the switching power supply 1, such an oscillation is repeated. In a steady state, the output voltage on the load side is divided by the resistors R6 and R7, and this divided detection voltage and the reference voltage of the shunt regulator Sr are compared. Then, the change of the output voltage is amplified at the shunt regulator Sr, the current flowing in the light-emitting diode PD of the photo coupler PC is changed, and in accordance with the quantity of light emission of the light-emitting diode PD, the impedance of the phototransistor PT changes. Thus, the charge and discharge of the capacitor C2 can be changed and the output voltage can be controlled so as to be constant.
In the conventional switching power supply 1 shown in FIG. 7, at light load the oscillation frequency increases and the switching loss is large, which is a factor lowering the circuit efficiency. In order to solve this problem, a method can be considered whereby on the output side of the switching power supply a circuit lowering the output voltage can be provided so that by changing the impedance on the output side the output voltage Vo is lowered.
In this case, by making use of a fact that the voltage VNB generated in the feedback winding NB of the transformer T decreases in proportion to the output voltage Vo, the degree of decrease in the output voltage Vo is adjusted. By lowering the voltage VNB, the gate voltage of the FET Q1 is made to oscillate in the range where the gate voltage does not reach the threshold and the turn-on of the FET Q1 is delayed, and by making the off-state period of the FET Q1 extended the oscillation frequency is lowered, and thus the switching loss is reduced.
However, such a switching power supply of RCC type is characterized in that at light load, the frequency increases because the output current is small, and when a circuit lowering the output voltage is provided as described above, all the output voltages are decreased, and accordingly when a constant output voltage is required, there is a problem that a constant-voltage control circuit is required.
Therefore, it is an object of this invention to provide a switching power supply where the increase of switching loss is suppressed and a constant output voltage can be obtained.
According to the invention, a switching power supply is provided having two or more DC outputs, the power supply comprising a DC power supply; a transformer having a primary winding, at least two secondary windings, and a feedback winding; a main switching element connected in series to the primary winding and to be turned on by a voltage generated in the feedback winding; and a rectifying circuit connected to the secondary winding, further comprising a starting circuit which turns on the main switching element at start of the power supply and a switching circuit provided between two outputs on the secondary side, and wherein by turning on the switching means, the voltage generated in the feedback winding is lowered during the off-state period of the main switching element, a voltage to be applied to a control terminal of the main switching element is controlled so as to be less than the threshold voltage, and the main switching element is turned on by the starting circuit.
According to the structure described above, the switching frequency can be lowered during standby and the loss can be reduced so that the switching circuit is provided between two outputs of secondary windings of a transformer, and whereby, by turning on the switching circuit, a voltage generated in a feedback winding during the off-state period of a main switching clement is lowered, and a voltage to be applied to the control terminal of the main switching element is controlled so as to be less than the threshold level.
The at least two outputs on the secondary side contain a low voltage output as a controlled output and a high voltage output as an uncontrolled output, and the switching circuit may be connected between the low voltage output and the high voltage output. In this case, the accuracy of the voltage of the low output voltage can be improved(increased).
Alternatively, the switching circuit may be connected between the high voltage output and the low voltage output. In this case, the accuracy of the voltage of the high output voltage can be improved.
In addition to the aforementioned structure, one output may be contained as an output on the secondary side and another output may be connected to the DC output through a rectifying element and the switching circuit connected to another secondary winding which is different from the output. In this way, a switching power supply can be made having a single output and the number of parts can be reduced.
The switching circuit may be turned on and off by a signal from the outside.
Alternatively, the switching circuit may be turned on by detection of lowered load power, so that thus changing signals from the outside becomes unnecessary Further the switching element may have a voltage control terminal or a current control terminal. In the case where the switching element contains a voltage control terminal, when the gate of the switching element is controlled so as to be less than the threshold level, a complete off state of the switching element is maintained and accordingly very little the loss is generated in the off state.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.